1. Field of the Invention
The present invention relates to a semiconductor laser device, an optical pickup and a fabrication method of a semiconductor laser device. More particularly, the present invention relates to a semiconductor laser device and optical pickup used in an optical disk system that carries out tracking control by the 3-beam method and a fabrication method of a semiconductor laser device.
2. Description of the Background Art
A conventional optical pickup carrying out tracking control by the 3-beam method is shown in FIG. 8A. Referring to FIG. 8A, a semiconductor laser chip 104 is mounted on a mount plane 102a of, for example, a stem 102. In stem 102 is mounted a photodetector 112 on an orthogonal plane 102b substantially orthogonal to mount plane 102a. The laser beam emitted from semiconductor laser chip 104 is divided into the 0th-order beam i.e., a main beam B0, and side beams B1 and B2 which are the ±first order beams. These three beams are collected on an information recording plane 116a of an optical disk 116. The three collected spots are disposed along a direction tilted several degrees (ψ) to the information track direction where the signal pits are aligned, as shown in FIG. 8B.
In the case where main beam B0 is located at the center of the information track which is a sequence of signal pits recording signals, the two side beams B1 and B2 are located at opposite directions about the center of the information track by equal distance. In other words, the two side beams cover the same area of signal pits. In contrast, when the main beam B0 is deviated from the center of the information track, the area of signal pits covered by the two side beams B1 and B2 will differ from each other.
The reflected light from optical disk 116 enters photodetector 112 through an optical element that has beam split capability such as a hologram 114b. Hologram 114b and a diffraction grating 114a are formed on a transparent substrate 114. This transparent substrate 114 is generally formed integrally with stem 102. Photodetector 112 is divided into a plurality of detector elements so that side beam B1 is detected by a photodetector element 112d whereas side beam B2 is detected by a photodetector element 112e, for example.
The position relationship between the information track and the main beam is detected as set forth below by photodetector 112 that detects the side beams. When the main beam is located at the center of the information track, the signals from the photodetector detecting the two side beams are equal in level. More specifically, the signal intensity Sd from photodetector element 112d is equal to the signal intensity Se from photodetector element 112e. The relationship of Sd=Se is established. When the main beam is offset from the center of the information beam, the signal of one side beam will become greater than the signal of the other side beam according to the offset direction. By detecting the difference (Sd−Se) of the signal levels of the side beams and adjusting the position or the like of an objective lens 115 having a beam-condensing function so that the difference becomes zero, the main beam can be maintained at the center of the information track. This is the mechanism of the tracking error control by the general 3-beam method.
Although the optical pickup employing the 3-beam method relies on the characteristic of the beam splitter, not all the light reflected from the optical disk enters the photodetector. The reflected light partially passes straight through the hologram to return to the proximity of the light emitting point X of semiconductor laser chip 104 as a main beam R0 and side beams R1 and R2, i.e. three return beams as shown in FIG. 8A and FIG. 9. These three beams are spaced apart from each other by the distance d of approximately 70-120 μm in the proximity at the side plane of semiconductor laser chip 104. The return side beam R1 incident on plane 102b orthogonal to semiconductor laser chip mount plane 102a of stem 102 is reflected at orthogonal plane 102b to return towards optical disk 116. This return side beam is diffracted at hologram 114b to directly enter photodetector elements 112d and 112e, or reflected at each plane of objective lens 115, the surface of optical disk 116, information recording plane 116a and the like. This reflected side beam is further diffracted by hologram 114b and directly enters photodetector element 112d or 112e to disturb the tracking control signal.
To eliminate such a problem, several conventional measures were taken. For example, with respect to R2: (a) the thickness of semiconductor laser chip 104 was adjusted so as to pass above the position of light emitting point X of semiconductor laser chip 104; and (b) the reflectance at the semiconductor laser chip end plane is reduced to 10% and below to prevent much of the light quantity from returning to the optical disk. Furthermore, the other side beam RI was caused: to (a) be incident upon stem 102 to be scattered; or (b) to be incident upon a submount (not shown) having a low reflecting film.
FIG. 9 shows an example of the above structure disclosed in Japanese Patent Laying-Open No. 62-52737 by the present applicant. The light emitting point X of semiconductor laser chip 104 is set to be in the proximity of the middle of the chip's height, i.e. approximately 50 μm from mount plane 102a of the stem. Since return side beam R2 is apart from return main beam R0 by a distance of approximately d=70-120 μm, side beam R2 will pass over semiconductor laser chip 104 and not return to the optical disk. In contrast, return side beam R1 will be incident on stem surface 102b. Therefore, the surface 102b of the stem is roughened for scattering. By this roughening process, the quantity of light that is reflected to return to the optical disk among the return side beam is significantly reduced. In practice, a tracking error signal of high reliability can be obtained by configuring an optical pickup using such a semiconductor laser. Stable tracking control can be achieved.
However, it has been identified recently that there is a case where the reflecting effect of return side beam RI at orthogonal plane 102b of the stem cannot be prevented sufficiently. Possible causes of such an event are set forth below.
(a) Various types of optical disks have been developed.
(b) A control method called differential push-pull (DPP) using side beams similar to the 3-beam method has been employed as the tracking control method. In the DPP method, the position relationship of the three beams, the relationship between the beam direction and the optical disk track, and the like differ from those of the 3-beam method.
(c) A high power laser of more than 50 mW at the end plane of the semiconductor laser chip is employed for the optical pickup of the information rewritable optical disk.
It is now necessary to further reduce the reflectance at orthogonal plane 102b of the stem to deal with the above-described causes. The following methods are known to reduce the reflectance at stem orthogonal plane 102b in addition to the plane roughening process.
(a) As shown in FIG. 10, an inclination portion 121 is provided at the stem orthogonal plane 102b where return side beam R1 strikes (Japanese Patent Laying-Open No. 61-250844).
(b) The light emitting point of the semiconductor laser is set upwards remote from mount plane 102a, and apply a low reflecting material at the end plane of the semiconductor laser chip at the side closer to the mount plane (Japanese Patent Laying-Open No. 62-18080).
(c) A nonreflective coating is applied on the stem (header portion) where side beam R1 strikes (Japanese Patent Laying-Open No. 61-250845).
These above methods allow the reflectance of the return side beam to be reduced significantly. For example, according to the structure of the above (a) shown in FIG. 10, return light R1 is reflected at inclination plane 121 provided at surface 113b of header 113 to be radiated as reflected light Rir out from the optical system. Therefore, light interference and the like will not occur at the optical system of the optical pickup. However, each of the above methods has an incidental problem. For example, the method of providing an inclination portion 121 at orthogonal plane 102b at the stem shown in FIG. 10 (Japanese Patent Laying-Open No. 61-250844) has difficulty in mass production. There was a fatal problem in productivity. This problem will be described in detail with reference to FIGS. 11A and 11B.
In general, header portion 113 is formed on stem 102 by press-molding. Specifically, header portion 113 is formed by pressing a warp-like (thin elongated sheet) iron material 120 with a die 151. In order to form a projection corresponding to head portion 113 from iron material 120 which is a flat sheet, an extremely strong stress must be applied to die 151. It is to be particularly noted that the portion 151b facing the leading end plane 113b (102b) of header portion 113 receives the greatest force in order to maintain leading end plane 113b planar. If the aforementioned inclination plane 121 is to be provided at the leading end plane 113b (102b) of header portion 113, die 151 must be formed with an inclination plane formation portion of a projection or recess corresponding to inclination plane 121. However, since this inclination plane formation portion corresponds to the region where the greatest force is applied, this inclination plane formation portion is easily susceptible to damage by the stress. Thus, the productivity thereof was extremely poor. In the worst case, this will induce damage of die 151 per se. In practice, it was impossible to provide stem 102 with header portion 113 having inclination plane 121 as described above directly formed by mass production.
Also, the method of applying a low reflective material at the end plane of the semiconductor laser chip at the closer side to the mount plane (Japanese Patent Laying-Open No. 62-18080) as well as the method of applying a nonreflective coating on the header portion (Japanese Patent Laying-Open No. 61-250845) causes the fabrication process of such a semiconductor laser device to become complicated, resulting in increase in the fabrication cost.